Electric fuse comprising plug terminals having an improved seal and pinning means



May 10, 1966 P. c. JACOBS, JR 3,250,879

ELECTRIC E COMPRISING PLU ERMINALS HAVING AN ROVED SEAL AND P ING MEANS Filed June 29, 1964 2 Sheets-Sheet 1 INV EN TOR.

BEN/WW ATTY.

PHILIP C. JACOBS, JR.

May 10, 1966 3,250,879

ELECTRIC NG AN P. C. JACOBS, JR FUSE COMPRISING PLUG TERMINALS HAVI IMPROVED SEAL AND PINNING MEANS Filed June 29, 1964 2 Sheets-Shea?I 2 INV EN TOR.

PHILIP C. JACOBS, JR.

MMNk ATTY.

United States Patent O HAVING AN IMPROVED SEAL AND PINNING MEANS Philip C. Jacobs, Jr., Newtonville, Mass., assigner to The Chase-Shawmut Company, Newburyport, Mass. Filed June 29, 1964, Ser. No. 378,561 Claims. (Cl. 20G-132) This invention relates to electric fuses for interrupting electric circuits in response to the ow of excessive currents therein.

From a more limited point of View this invention relates to electric fuses having casings which are closed on both ends by terminal plugs. Such fuses are widely applied in the art. Fuses of this description are fully disclosed in U.S. Patent 2,658,974 to Frederick J. Kozacka, Nov. 10, 1953 High Current Carrying Capacity Current-Limiting Fuse, assigned to the same assignee as the present invention. Reference may be had to the aforementioned patent in regard to additional details of electric fuses of the above description which details have no immediate bearing on the present invention.

In fuses whose casings are closed by metal plugs which have a relatively large heat absorbing capacity, the problem arises how to prevent the escape of products of arcing and to prevent the loss of pulverulent arc-quenching filler through the line annular gap formed between the outer lateral surface of the plug and inner lateral surface of the casing.

This task rmay be achieved by maintaining close tolerances and fitting the plugs under considerable pressure into the casing. Maintaining suiiiciently close tolerances for the purpose in hand greatly increases the cost of production.l If the plugs are fitted under considerable pressure into the casing, the latter is subjected to considerable stresses. This isnot desirable. The presence of high stresses calls forthe use of casing materials which are relatively expensive and excludes the use of other casing materials which are fully adequate for the purpose in hand, except for the fact that it is impossible to insert under considerable pressure terminal plugs into casings of such materials. This is particularly true in regard to inorganic heat resistant casing materials such as, for instance, ceramic materials.

It is, therefore, one object of this invention to provide electric fuses having terminal plugs at the ends of the casings thereof, which fuses are not subject to the aforementioned drawbacks and/or limitations.

Electric fuses whose casings are closed by a pair of terminal plugs may also be sealed effectively by appropriate annular seals of an elastomer such as, for instance, so-called O-rings.

This calls, however, for additional manufacturing steps which must be performed by relatively skilled labor.

It is, therefore, another object of this invention to provide electric fuses including terminal plugs and annular seals which can more readily and more easily be installed than so-called O-rings, and which can readily be installed by semi-automated or automated manufacturing equipment.

When interrupting rnajor fault currents high transient pressures are generated inside of the casing of fuses. These pressures act upon the axially inner surfaces of the terminal plugs and tend to drive the latter out of the ELECTRIC FUSE COMPRISIN G PLUG TERMINALS p ICC casings. Steel pins are generally driven transversely across the casings into the terminal plugs to maintain the latter plugs in position against the action of high internal pressures. The higher the dynamic pressure inside of the casing, the larger the number and the size of steel pins required to maintain the terminal plugs in their position. The larger the number and the size of the steel pins, the-larger the number and size of the pinreceiving radial holes in the ends of the casing, the smaller the mechanical strength of the casing to withstand the shearing action imposed upon the casing by the presence of the steel pins. This, in turn, makes it necessary to use relatively expensive casing materials having a relatively great wall thickness and a relatively high shearing strength.

It is, therefore, another object of this invention to provide electric fuses having plug terminals wherein the plug terminals are firmly held in position and wherein the mechanical integrity of the ends of the casing is maintained to a far higher extent than in any comparable prior art fuses.

Another object of the invention is to provide electric fuses having plug terminals and including an element having the dual function of sealing the ends of the casing and of forming pins for pinning the plug terminals to the casing.

Another object of the invention is to provide electric fuses having plug terminals and including means, in addition to pins, for maintaining the plug terminals in position, which means make it possible to significantly reduce A the number and the size of the pins required to maintain the plug terminals in position inside of the casing.

Another object of the invention is to provide fuses having casings of inorganic temperature resistant materials, which fuses are closed by plug terminals firmly held in position without subjecting the casings to any radial pressure tending to cause bursting of the casings, and the plug terminals being firmly held in position without subjecting the casing to any shearing action incident to blowing of the fuses.

Another object of the invention is to provide fuses having casings which are rectangular-in cross-section having plug terminals which are rectangular in cross-section FIG. 3 is an end view of the structure of FIGS. 1 and 2;

FIG. 4 is a cross-section of a detail 'shown on large scale than FIGS. 1 3;

FIG. 5 is a section similar to that shown in FIG. 2 of a modification of the structure of FIG. 2; and

FIG. 6 is a section along -6-6 of FIG. 5.

Referring now to the drawings and more particularly to FIGS. 1-3 thereof, reference numeral 1 has been applied to indicate fusible element means as, for instance, perforated ribbons of silver or copper. The fusible element means l. are surrounded Iby a pulverulent arcquench iiller 2. Filler 2 is preferably quartz-sand to maxa much imise the arc voltage generated incident to blowing of the fuse. The fusible element means 1 and the arc-quenching ller 2 are housing in a substantially tubular casing 3. Casing 3 may be made of a synthetic-resin-glass-cloth laminate as, for instance, a melamine glass-cloth laminate. It may, however, be perferable to make casing 3 of an inorganic and more heat resistant material than a melamine-glass-cloth laminate. Such a material is relatively porous precompressed asbestos liber cement, frequently referred to as densifed asbestos `fiber cement. A pair of terminal plugs 4 is inserted into the ends of casing 3. FIGS. l `and 2 show but one end of casing 3 and but one of the terminal plugs 4. Both ends of casing 3 and both terminal plugs 4 are identical, and for this reason but one of the ends of casing 3 and but one of the terminal plugs 4 have been shown in FIGS. l and 2. The plugs 4 inserted into the ends of casing 3 are conductively interconnected by the fusible element means 1. To this end the axially inner end surfaces of terminal plugs 4 are provided with grooves 5 receiving the axially outer ends of fusible element means 1. The grooves S are filled with soft solder (not shown) to minimize the resistance between terminal plugs 4 and fusible elements 1. Each plug 4 is provided with a knife blade contact 6 for connecting the fuse structure into an electric circuit.

In the particular structure shown in FIGS. 1-3 the cross-section of the outer surface as well as that of the inner surface of casing 3 is rectangular. Plugs 4 are provided with grooves 7 extending around the periphery of plugs 4, i.e. grooves 7 are substantially rectangular having predetermined radii at each of the four corners. If desired, a groove may be provided in casing 3 juxtaposed to each of the grooves 7 in plugs 4 and complementary to each of the grooves in plug 4. Such complementary grooves in casing 3 have been indicated in FIGS. 1-3. In most applications the presence of complementary grooves in the ends of casing 3 is unnecessary, and may even be undesirable, since their presence lessens the mechanical strength of casing 3.

`Casing 3 defines passageways y8 projecting transversely across 'its lateral wall adjacent each of both ends thereof. The structure of FIGS. l to 3 is provided with three such passageways 8 adjacent each end thereof. The inner ends of passageways 8 are juxtaposed to the grooves 7 in terminal plugs 4 and passageways yS are arranged at right angles to the immediately adjacent portions of grooves 7.

An appropriate sealing medium may be introduced i If the clearance between casing 3 and plug 4 is relatively large, the sealing medium flows out of groove 7 into the hair gap formed Ibetween parts 3 and 4 and seals that gap.

There are many compounds that lend themselves to be used as sealing medium. If casing 3 is made of a synthetic resin, or a synthetic resin laminate, a compatible synthetic resin should be injected into the structure to form a seal. If casing 3 is made of a porous inorganic substance a metal such as, for instance, soft solder may be introduced under pressure into the lower passageway 8. A die casting machine may readily be adapted for the purpose in hand. Upon curing of the synthetic resin injected into the lower passageway 8 or solidilication of melted metal injected into that passageway a pair of annular seals 9 is formed of which each is coextensive with the periphery of one of terminal plugs 4. Each seal 9 includes integral projections 10 extending into passageways `8 defined by casing 3. The annular seal with its projections i0 is a dual function structure. The annular portion of the structure performs as a seal precluding the emission of hot products of arcing from casing 3 and precluding leakage of arc-quenching filler. The pinshaped projections 10 operate as pins maintaining plugs 4 in position against the action of pressure build-up inside of casing 3. If casing 3 is made of a porous material such as, for instance, precompressed or densified asbestos fiber cement, the sealing medium under pressure penetrates more or less deeply into the pores of the casing material. This establishes a firm bond between the casing and the annular seal 10 and, therefore, the latter tends to firmly maintain terminal plugs 4 in their respective position. This way of securing terminal plugs to casings of fuses is far superior to the conventional pinning method because (a) It does not impair the mechanical integrity of the ends of the casing as any conventionalmulti pin structure does; and

(b) The area of the interfaces at which pressure is transmitted from terminal plug 4 to annular seals 10 and at which pressure is transmitted from annular seals 10 to casing 3 is relatively large and, therefore, the specific pressures at these interfaces are relatively small.

In drawing FIG. 4 it was assumed thatthe inner surface of casing 3 is lsufficiently porous as, for instance, that of precompressed asbestos `fiber cement, or densiiied asbestos fiber cement, to allow a significant penetration of a sealing substance into it. Terminal plugs 4 are provided with semicircular grooves 7, but no juxtaposed grooves are provided in casing 3. The semicircular seal 9 ills groove 7 and extends into the toroidal hair gapshown in exaggerated form-existing between casing 3 and terminal plug 4.

It will be apparent from FIG. 3 that each groove 7 defines two vertical passageways, i.e. a left passageway and a right passageway, wherein a sealing substance may rise that had been injected into lower passageway 8 in casing 3. The aforementioned left and right passageways are communicating vessels and, therefore, the level of any sealing substance is the same in both passageways, provided that the sealing substance is, or behaves substantially like, a liquid. If the viscosity of the sealing substance is particularly high, the sealing substance may rise more rapidly either in the right or left passageway, or even leave one of the two passageways formed by groove 7 more or less empty. The structure of FIG. 3 is provided with two upper outlets 8 to make it possible to check whether or not both the left and the right side of groove 7 have been filled with the sealing substance. If the sealing substance is a liquid subject to the laws of communicating vessels, both upper passageways 8 may be merged into one single upper passageway.

In FIGS. 4 and 5 the same reference numerals with a prime added have been applied to indicate like parts as in FIGS. 1-3. Thus 1 are fusible element means surrounded by a pulverulent arc-quenching filler 2. Fusible element means 1 and filler 2 are housed in casing 3' of precompressed asbestos fiber cement, or densified asbestos liber cement. Cylindrical plugs 4 having fuse-link-receiving grooves 5' are provided with peripheral grooves 7 which may be helical, if desired. Casing 3 defines a pair V of passageways 8 at each end thereof angularly displaced deg. A nozzle lll is shown to be inserted into one of passageways for injecting a sealing medium into one of grooves '7'. part of each of both terminal plugs 4', of which but one is shown in FIG. 5. As shown in FIG. 6 the annular casting 9' has radial projections or pins 10 forming an integral part thereof and extending into the two passageways 8 deiined by casing 3. In the structure of FIGS. 5 and 6 two projections or pins 10 are suliicient to firmly position each annular seal 9 and terminalplug 4 because the former is held in position by the penetration of the sealing medium, e.g. soft solder, into the inner cylindrical surface of casing 3', and because seals 9 and plug 4 are mechanically interlocking. Casing 3 is formed by a relatively porous prismatic block of preompressed Knife blade contact 6 forms an integral i asbestos liber cement having a cylindrical bore receiving fusible element means 1 and quartz sand fllller 2.

Where the bonding action of the sealing substance is relatively small, e.g. Where the inner surface of the casing 3' is relatively smooth, it may be necessary, or desirable, to provide additional plug-fastening-pins aside from the plug-fastening-pins formed by, and integral with, annular casting 9.

In FIGS. 5 and 6 the annular seal or casting 9 is provided with but two integral fastening pins 10', whereas in FIG. 3 the endless sealing body 9 is provided with three integral fastening pins 10. It will thus be apparent that the number of integral fastening pins may be increased, if desired, as may be necessary, or desirable, in any particular application.

The advantages of using precompressed asbestos fiber cement, or densified asbestos fiber cement, as a casing material for electric fuses have been set forth in the copending patent applications of Erwin Salzer et al., Ser. No. 355,804, filed March 30, 1964, for Electric Cartridge Fuses and of Frederick J. Kozacka et al., Ser. No. 376,417 filed June 19, 1964, for High Interrupting Capacity Fuses Having Casings of Inorganic Material, both assigned to the same assignee as the present invention. The aforementioned patent applications are not concerned with fuses having plug terminals. It was heretofore virtually impossible to use plug terminals in connection with fuse casings of inorganic materials such as, for instance, steatite and other conventional temperature resistant casing materials. As is apparent from the foregoing the present combined sealing and pinning structure lends itself particularly well to be used in connection with casings of precompressed asbestos fiber cement, or densified asbestos fiber cement, on account of the porosity of that material which is sutiiciently high to effect a firm bond with the integral sealing and pinning member, but suiiiciently low to safely preclude any escape of hot products of arcing across and through the lateral wall of the casing.

It will be apparent from the foregoing that the composite sealing and plug pinning structure according to my invention may be a die casting which remains permanently in place in its casting die, or an injection molded part which remains permanently `in place in its molding die.

It will be understood that I have illustrated and described herein two preferred embodiments of my invention, and that various alterations may be made therein without departing from the spirit and scope of the appended claims.

I claim as my invention: 1. An electric fuse comprising in combination: (a) fusible element means; (b) a pulverulent arc-quenching ller surrounding said fusible element means;

(c) a substantially tubular casing housing said fusible element means and said arc-quenching filler, said casing dening passageways having a predetermined spacing from the ends of said casing projecting transversely across said casing;

(d) a pair of terminal plugs each inserted relatively loosely into one of the ends of said casing, each establishing an annular gap vbetween the outer surface thereof and the inner surface of said casing, and each forming an annular groove in said outer surface thereof, said pair of plugs being conductively interconnected by said fusible element means; and

(e) a pair of annular seals each coextensive with said annular groove in one of said pair of plugs and each interposed between said casing and one of said pair of plugs, each of said pair of seals having a spacing from said ends of said casing equal to said predetermined length and each including integral projections extending into said passageways defined by said casing.

2. An electric fuse comprising in combination:

(a) fusible element means;

(b) a body of quartz sand surrounding said fusible element means;

(c) a substantially tubular casing of an inorganic insulating material housing said fusible element means and said body of quartz sand, the inside of the'lateral wall of said casing being porous and said lateral wall of said casing defining passageways projecting transversely across said lateral wall of said casing adjacent each of both ends thereof;

(d) a pair ofterminal plugs each inserted into one of the ends of said casing, said pair of plugs being conductively interconnected by said fusible element means, eachof said pair of plugs defining an endless groove coextensive with the periphery thereof; and

(e) a pair of annular seals each coextensive with said groove in one of said pair of plugs and having projections extending into said passageways defined by said casing, each of said pair of seals having additional relatively small projections at the side thereof remote from one of said pair of plugs mating with the pores on the inside of said lateral Wall of said casing and there-by establishing a bond between each of said pair of seals and'said lateral 'wall of said casing.

3. An electric fuse comprising in combination:

(a) fusible element means;

(b) a, body of quartz sand surrounding said fusible element means;

(c) a substantially tubular casing of porous precompressed asbestos liber cement housing said fusible element means and said body of quartz sand, the lateral wall of said casing defining passageways projecting transversely across said lateral wall adjacent each of both ends'thereof;

(d) a pair of terminal plugs each inserted into one of the ends of said casing, said pair of plugs being conductively interconnected by said fusible element means, and each of said pair of plugs defining a groove coextensive with the periphery thereof; and

(e) a pair of endless solidified castings each having one side projecting into and substantially filling a groove in one of said pair of plugs and each having at the side opposite to said one side integral projections extending into said passageways defined by said lateral wall of said casing and thus pinning each of said pair of plugs to said casing, said side of each of said pair of castings opposite to said one side further including a rough surface having relatively fine projections mating with the pores defined by the inner surface of said lateral wall of said casing.

4'. An electric fuse comprising in combination:

(a) fusible element means;

(b) a body of quartz sand surrounding said fusible element means;

(c) a prismatic block of porous precompressed asbestos fiber cement having a cylindrical bore housing said fusible element means and said body of quartz sand, said block defining passageways projecting transversely across the lateral Wall of said block and bounded on one end by the cylindrical surface of said bore and on the other end by a pair of planes defined by sides of said block;

(d) a pair of cylindrical terminal plugs each inserted into one of the ends of said bore, said pair of plugs being conductively interconnected by said fusible element means and each of said pair of plugs defining an annular groove coextensive with the periphery thereof;

(e) a pair of annular solidified castings each having a radially inner side projecting into and substantially filling said annular groove in one of said pair of plugs and each having a radially outer side including integral projections extending into said passageways defined by said lateral Wall of said block and thus pinning each of said pair of plugs to said block, said radially outer side of each of said pair of castings having relatively ne surface projections mating with the pores deiined by said lateral wall of said block.

5. An electric fuse as specified in claim 1 wherein the outer'surt'ace and the inner surface of said casing are rectangular in cross-section, said terminal plugs are rectangular in cross-section, each of saidv pair of terminal plugs being provided on `the outer surface thereof with References Cited by the Examiner UNITED STATES PATENTS 2,837,614 v 6/1958 Fister 20G-120 2,929,900 3/1960 White 200-131 FOREIGN PATENTS 456,583 11/ 1936 Great Britain.

BERNARD A. GILHEANY, Primary Examiner. 

1. AN ELECTRIC FUSE COMPRISING IN COMBINATION: (A) FUSIBLE ELEMENT MEANS; (B) A PULVERULENT ARC-QUENCHING FILLER SURROUNDING SAID FUSIBLE ELEMENT MEANS; (C) A SUBSTANTIALLY TUBULAR CASING HOUSING SAID FUSIBLE ELEMENT MEANS AND SAID ARC-QUENCHING FILLER, SAID CASING DEFINING PASSAGEWAYS HAVING A PREDETERMINED SPACING FROM THE ENDS OF SAID CASING PROJECTING TRANSVERSELY ACROSS SAID CASING; (D) A PAIR OF TERMINAL PLUGS EACH INSERTED RELATIVELY LOOSELY INTO ONE OF THE ENDS OF SAID CASING, EACH ESTABLISHING AN ANNULAR GAP BETWEEN THE OUTER SURFACE THEREOF AND THE INNER SURFACE OF SAID CASING, AND EACH FORMING AN ANNULAR GROOVE IN SAID OUTER SUR- 